In the telephone communications cable there is always a degree of interference between circuits due to the proximity of paired circuits. The induction of "disturbing" pair currents into the "disturbed" pair, i.e. "crosstalk", is a primary form of interference. Two currents are carried by what has been termed the "disturbed" pair; the transmission information current and the unwanted current induced in pairs in close proximity. This secondary current superimposed on the transmitted signal results in unreliable signal results as well as a reduction in signal clarity.
With the advent of pulse code modulation, (PCM), in the field of transmission techniques on balanced pairs, (potential symmetrical to ground), it became necessary to divide the number of pairs in the cable core into two groups of equal number, one group carrying signals in one direction and the other, in the opposite direction, or by the use of two separate cables. This latter technique, though sound from the engineering level, is at times more expensive, especially if pair sizes (pair counts) of cables are small.
Since present day cables are made of prestranded units, the conventional method of stranding first one half of the cable pairs, applying shield and then stranding over it the second half of the cable pairs was not practical, (i.e. the prestranded units could not be assembled in symmetrical concentric fashion). The earlier solution consisted of an "S" or "Z" shaped internal shield. This was followed by the concept of the "D" shaped internal shield, which resulted in improved shielding efficiency, with respect to the earlier "S" or "Z" concepts, and the minimum requirement imposed by the PCM transmission systems known as T-1 carrier.
Since present day internal shields are closely coupled capacitively to the overall shield, it can be assumed to be at ground potential, though no direct ground path exists. This results in the need for high voltage separation of the working circuits and the internal shield. One type of internal shield is a laminated tape consisting of a sheet of aluminum bonded with a pressure sensitive adhesive to a sheet of polyester on each face, with an overlap of polyester on each edge of the aluminum. These overlapping edges of polyester are bonded to each other with adhesive. This type of internal shield has two distinct disadvantages. One is that there is an air gap at each edge of the aluminum where the polyester coatings taper toward one another beyond the edge of the aluminum. This separation of the polyester strips at the edges of the aluminum is equal to the thickness of the aluminum.
The gap formed between the edge of the aluminum and the location where the polyester strips come together provides a space for water ingress which will corrode the internal aluminum shield and it makes it difficult to stop a gas flow along the conductor when pressure dams are constructed. These gaps also allow for the influx of the cable filling compound, which has a tendency to delaminate this variety of internal shield tape.
The present invention provides an improved shield which is formed by bending the edge portion on each side of a strip back on itself to provide 180.degree. curves along the edges instead of the abrupt corner which is obtained with commonly used aluminum strips. The improved shield can be made by folding a metal strip along its center region but this leaves the opposite edge with a square configuration; so the folding of both edges is a superior construction. The strip can be folded so that the edges are folded over on the same side of the strip and at locations which bring the edges into abutment, or substantial abutment after folding. When made into a D-Screen, the edges of the original strip are located away from the shielded group of pairs.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.